Ab initio chemical kinetics for the NH₂ + HNO_x reactions, part I: Kinetics and mechanism for NH₂ + HONO

The kinetics and mechanism for the reaction of NH₂ with HONO have been investigated by ab initio calculations with rate constant prediction. The potential energy surface of this reaction has been computed by single-point calculations at the CCSD(T)/6-311+G(3df, 2p) level based on geometries optimized at the CCSD/6-311++G(d , p) level. The reaction producing the primary products, NH₃+NO₂, takes place via precomplexes, H ₂N···c-HONO or H ₂N···t-HONO with binding energies, 5.0 or 5.9 kcal/mol, respectively. The rate constants for themajor reaction channels in the temperature range of 300-3000 K are predicted by variational transition state theory or Rice-Ramsperger-Kassel-Marcus theory depending on the mechanism involved. The total rate constant can be represented by k_total =1.69×10^-20 × T ^2.34 exp(1612/T ) cm ³ molecule^-1 s^-1 at T =300-650 K and 8.04×10^-22 × T ^3.36 exp(2303/T ) cm ³ molecule^-1 s^-1 at T =650-3000 K. The branching ratios of the major channels are predicted: k₁ +k ₃ producing NH₃ +NO₂ accounts for 1.00-0.98 in the temperature range 300-3000 K and k₂ producing OH+H₂NNO accounts for 0.02 at T >2500 K. The predicted rate constant for the reverse reaction, NH₃ +NO₂→NH₂ +HONO represented by 8.00×10^-26 × T 4.25 exp(-11,560/T ) cm³ molecule^-1 s^-1, is in good agreement with the experimental data.